Because of the wording during sex education (i.e. your parents have sex when they sleep together) that they would really be asleep and that dad's member would stir, raise it's ugly head (haha!) and plunge itself where we don't talk about on our mama, only on yo mama!
A childhood belief about sex

others believed things like this
- My mother's name was Anna. She was called "Annemor"(Duck mama). I was convinced I was a duckling.— Christine Norway
- When i was a kid i used to think that all ugly people were imported from space haha!!!!!— Abbey Jackman Mulligan
- I used to think it was true that only ugly girls poop and I thought I was an ugly girl because I poopedAnon
- My wife used to believe that the purpose of the stick shift in her Dad's car was to stir the gas.— Curt
- I used to believe that my parents and sister were alligators underneith there zip on human costume...haha— Alyssa
- i used to think when you sleep your eyes turened round to watch performances in your head— rasverone